In general, a magnetic resonance image obtained by means of a monitor scan is called a ‘monitor image’ and a magnetic resonance image obtained by means of an imaging scan is called an ‘MR image’.
The nuclear spin of a patient's body tissue situated in a static magnetic field generates a magnetic resonance signal (“MR signal”) when excited by a radio frequency excitation pulse (“RF pulse”) having the corresponding Larmor frequency, and magnetic resonance images of the patient are obtained by means of this MR signal. A large amount of diagnostic information, such as anatomical diagnostic information, and also biochemical information and functional diagnostic information, and the like, can be obtained from these magnetic resonance images, and therefore magnetic resonance imaging apparatuses have become indispensable in the field of present-day medical diagnostic imaging.
A contrast agent is sometimes used in order to capture moving images of blood vessels using a magnetic resonance imaging apparatus of this kind. A contrast agent increases the contrast of the images. According to this method, the contrast agent is injected into the blood vessels of the patient, and when the contrast agent reaches the image region of interest, imaging scans are taken in order to acquire MR images of the image region of interest. In this method, it is important that the acquisition of the MR signal starts at the timing at which the contrast agent enters into the image region of interest. However, since the flow rate of the contrast agent differs according to the patient, it is difficult to obtain this timing.
One method for obtaining this timing is fluoro-triggered-enhanced MRA (Magnetic Resonance Angiography). This is an imaging technique wherein monitor scans are performed after the contrast agent has been injected into the patient's blood vessels. Before taking imaging scans of the image region of interest of the patient, monitor scans are performed in a monitor region, which is a separate region from the image region of interest, situated upstream in the blood flow passing through the blood vessels of the image region of interest. In this monitor scanning, a plurality of images are generated in consecutive fashion and displayed in sequence on a monitor. The operator is able to observe the state of flow of the contrast agent into the monitor region, in real time, by means of the displayed monitor images, and hence he or she can ascertain the timing at which the contrast agent arrives at the diagnostic region.
The images of the monitor region displayed as monitor images may be magnetic resonance images obtained after the contrast agent has been injected, or they may be difference images obtained by subtraction of a magnetic resonance image before contrast agent injection and a magnetic resonance image obtained after the contrast agent has been injected.
Since the monitor images are two-dimensional magnetic resonance images having a prescribed slice thickness, they do not provide satisfactory dynamic imaging of the state of travel in a blood vessel which extends three-dimensionally. On the other hand, if the slice thickness is set to a large thickness of 20 mm to 30 mm in order to include the state of travel in the blood vessel, then equalization of the MR signals in the slice direction occurs, the contrast of the blood vessel declines, and hence the blood vessel becomes difficult to identify.
Therefore, the operator becomes unable clearly to identify the flow of the contrast agent. As a result, problems may arise in that the operator misses the timing at which to instruct performance of imaging scans, and hence the desired MR images cannot be acquired.